Here is the issue with OTLs and speaker loads. There are two major concerns. The first is this:
http://www.atma-sphere.com/papers/paradigm_paper2.html
In short you want a speaker that is designed for tubes. The second is that the smaller you make the OTL, the more it will want higher impedances to really perform right.
A lot has to do with the circuit of the amp and you should check with the manufacturer on this, but in general you will find that the triode-based OTLs can driver lower impedances as their output impedances are lower. Now this brings on a discussion of loop negative feedback, which is a common method of lowering the output impedance on any amp, but here is something everyone should know:
The addition of negative feedback will not allow the amplifier to make more power into lower impedances.
This is because loop feedback really doesn't change output impedance! If it really did, feedback would give you greater power at lower impedances, and it doesn't.
There is an additional factor that comes into play here. Put plainly: There is no argument for 4 ohms or less in high end audio, if **sound quality** is your goal. No matter what kind of amplifier you have, transistor, class D, transformer-coupled tube amp, SET or OTL, the simple fact is that all of them sound better at higher impedances simply due to the fact that they will make less distortion in one way or another. In the case of transistor/class D amps, you get less odd-ordered harmonics, which BTW the ear finds to be very objectionable. In the case of all tube amps, you get less lower-ordered harmonics (2nd, 3rd, 4th) which the ear interprets as 'bloom', 'warmth', 'thick', etc. these lower orders mask detail so if they are gone the system is instantly more transparent without additional brightness (brightness is associated with the 5th, 7th and 9th harmonics).
So the bottom line is you want a higher impedance in the speaker if you can get it, all other things being equal.
Now OTLs, while having an economy of scale, do not suffer the way most other amps do when you make a bigger version. If the designer has done his homework, bigger OTLs will never loose PRat; they will be just as nimble as a smaller amplifier without loosing detail. The other advantage of OTLs is that if built right, they do not suffer crossover or notch distortion issues at the zero crossing point- the only other amps like that are SETs. The point here is that you can get a bigger amp to drive lower impedances if needed.
Now with respect to the impedance curve of the speaker: it is a common myth that OTLs need a *flat* impedance curve. That's not quite true- if the speaker impedance is too low for the amp, then yes, that's a good idea (and it usually is anyway). But otherwise, what an OTL is **really** looking for is that the entire impedance curve be high enough- often that is a lot more important than a flat impedance curve.
A good example is the Sound Lab ESL. This speaker has an impedance curve based on a capacitor and some inductors and has nothing to do with box resonance. This is why transistors have such a hard time with this speaker- its as high as 50 ohms in the bass and only 1.5 to maybe 3 ohms at 20KHz. That's quite a spread. Now a transistor amp can make lots of power in the highs, but hardly any in the bass. So they tend to be bright with no bass or just seem like they can't drive the speaker. You put an OTL on there and suddenly the speaker has bass authority and extension that the transistors could not do, and with an OTL that might be 1/3 the power of the SS amp will make the same volume.
There is an outboard device called the ZERO that is an excellent problem solver for tube amps (OTLs in particular) if you simply *have* to use a 4-ohm speaker. The most common question we get is 'doesn't that obviate the OTL quality?' and the answer is No. The ZERO loads the amp at 16 ohms and has only provisions for 4,3 and 2 ohms. The turns ratio is so low that it has bandwidth from 2Hz to 2MHz, exceeding the bandwidth of nearly any amp its hooked up to. It also does not block DC as its an autoformer. This sort of thing requires that you have an amplifier that already has a fairly low output impedance- you could never do this with a conventional output transformer!
